1. Field of the invention
The present invention relates to an aromatic polyimide film provided with protruded and depressed portions on the surface.
2. Description of prior art
A polyethylene terephthalate film has been heretofore used as a substrate (i.e., support) of a magnetic recording medium, because the polyethylene terephthalate film is excellent in various properties such as heat resistance, mechanical strength and surface smoothness, and further is obtainable at a low cost. Recently, a higher density recording system, prolonged recording period, higher quality of recorded image or a smaller sized and light-weight medium are demanded, and hence a magnetic recording medium comprising a thin layer of a metallic material such as Co, Cr, Ni and Fe or an alloy thereof provided on a film support by utilizing sputtering or vacuum deposition has been widely developed.
When the metal thin layer is formed on the film support by the process such as sputtering, the temperature of the film rises over 200.degree. C. due to an energy given by the collision of the metal particles to the film, even though such process is performed at room temperature without heating the film. Further, depending upon the nature of the employed metal, the film support may be heated at a temperature of higher than 200.degree. C. to form a metal thin layer thereon in order to improve magnetic properties such as coercive force or to improve adhesion between the thin layer and the support. Accordingly, as the film support of a magnetic recording medium in which a magnetic recording layer is formed by the process such as sputtering or vacuum deposition, the polyethylene terephthalate film is not satisfactory because the film has not appropriate heat resistance. There is a process where a magnetic recording layer is formed on a film support having poor heat resistance such as a polyethylene terephthalate film by sputtering and the like, under the condition that the film support is cooled during the sputtering and the like. However, this method is disadvantageous, because the obtained magnetic recording layer sometimes decreases in the magnetic properties or shows unsatisfactory bonding strength.
For the above reasons, employment of an aromatic polyimide film having high heat resistance as a film support of a magnetic recording medium has been proposed.
The aromatic polyimide film, particularly an aromatic polyimide film obtained by reaction of an aromatic carboxylic acid compound mainly containing a biphenyltetracarboxylic acid and an aromatic diamine compound shows resistance to high temperatures of higher than 300.degree. C., so that the aromatic polyimide film is considered to be favorable as a film support of a magnetic recording medium utilizing a metallic recording layer (i.e., magnetic recording medium having a magnetic recording layer formed by metallizing, etc.).
It is generally known that various properties of magnetic recording media such as electromagnetic conversion characteristics, smooth running properties and running endurance (or durability) greatly depend on the surface conditions of the magnetic recording layer. Hence, studies for improvements of the surface conditions of the magnetic recording layer have been earnestly made.
A metallic recording layer of a magnetic recording medium generally has a thickness of approx. 1,000 to 5,000 angstroms, and the layer is prominently thinner than that of the coated magnetic recording layer. Hence, the surface of the magnetic recording layer shows almost the same conditions (or shape) as those of the surface of the film support. Accordingly, provision of a favorable shape to a surface of the film support makes it possible to improve various properties of the magnetic recording layer (i.e., metal thin layer) such as electromagnetic conversion characteristics, smooth running property and running endurance. More specifically, if the surface of the film support is too rough, the distance between the magnetic recording layer and a magnetic head becomes too large in the recording and reproducing procedures, whereby the output level lowers due to so-called spacing loss. Further, existence of coarse protrusions on the surface of the film support causes drop-out phenomenon. On the contrary, if the surface of the film support is too smooth, there arise other problems although electromagnetic conversion characteristics do not deteriorate. For instance, the film shows a poor running property in the film-forming procedure or film-processing procedure. Further, when the film is used as a support of a magnetic recording medium such as a magnetic tape or a floppy disc, the resulting medium is markedly deteriorated in smooth running, running endurance, recovery from clogging on a head, etc.
For coping with the above-mentioned problems relating to running properties of a magnetic recording medium, there has been heretofore proposed a method of incorporating fine particles of an inorganic or organic material into the film support so as to provide a great number of small sized protruded and depressed portions on the surface of the film.
According to the studies of the present inventors, however, they have found that the heretofore proposed method of incorporating fine particles of those materials into the film support cannot provide such extremely small sized protruded portions as enable to decrease spacing loss. It has been also confirmed that relatively large sized protruded portions are formed and the diameter distribution of the protruded portions is wide, although the average diameter is small. Hence, occurrence of troubles in the electromagnetic conversion characteristics such as occurrence of drop-out phenomenon is not well prevented by the method.